The present invention relates generally to optical lens systems, and more particularly to lens systems suitable for endoscopes and the like.
In endoscopy and related fields, such as borescopes and dental scopes, the complete optical system is thought of as consisting of four basic and separate optical functions. Those functions are, in sequence of the direction of the travelling light, as follows:
1. an objective which forms the first image of an object under surveillance,
2. a field lens which images the pupil of the objective onto the next image transfer lens,
3. an image transfer lens which reimages the first image onto the next field lens. The pupil and image transfer steps are repeated as often as is needed to obtain a desired tube length, and
4. a focussing lens which presents the final image to a sensor, like a person""s eye, a CCD camera, or a photographic film.
This approach is the classical approach, and it is appropriate for the following reasons:
1. The design of the optical system is broken up into parts with single and clearly defined and separate functions, functions to each of which an optical designer may bring considerable experience, and
2. The light transfer capacity and information transfer capacity of an endoscope is at a maximum when the optical power is concentrated at the image planes and pupil planes. The expedience of this approach is brought out by numerous
U.S. patents on endoscopes which consistently treat the objective, the relay system, and the eyepiece as separate parts of the total system.
The disadvantage of treating the different optical components as separate entities is that the distribution of the optical powers is very uneven and that certain aberrations are naturally at a maximum, like astigmatism, field curvature, and chromatic aberrations. The correction of these aberrations require relatively short radii. These short radii are difficult to fabricate, require tight tolerances, and they are therefore the main contributors to the considerable cost of the fabrication of an endoscope. A truly inexpensive endoscope, sufficiently inexpensive to be offered as a disposable item, is presently not practical with conventional designs.
The present invention provides an integrated optical system suitable for endoscopes, borescopes, dental scopes, and the like which contains a minimum of elements and which elements have relatively long radii and need not be of a meniscus shape. The outside entrance pupil location is very suitable for a tapered probe or for concealment. The entrance pupil distance sufficient to accommodate a line-of-sight deviating prism is a natural consequence of the arrangement of the optical groups. The system leads itself to mass production and is highly insensitive to tilt and decentration of its components. As a consequence it is eminently suitable as a disposable item.
Broadly, the foregoing advantages are achieved in a lens system which is characterized by an integrated design which has an external entrance pupil and in which the majority of the groups are displaced from the image planes and pupil planes. In this way most components share in the pupil transfer as well as in the image transfer. Moreover, the aberration correction is distributed in an advantageous way over all the groups, providing relief to the first group which conventionally is in need of most of the aberration correction. It has been found that this integration of the optical functions and aberration correction is very beneficial in that it greatly simplifies the optical system.
A plano-convex lens, or even a double convex lens when used according to the invention can be corrected for astigmatism since it is displaced from the stop location. In this way no optical surfaces of very short radii are needed to correct the astigmatism of the total optical system. Furthermore, the spherical aberration of a convex-plano lens used in the present invention is very near the minimum possible for a single element. Also the chromatic aberration is greatly reduced by the displacement of the elements from the image planes and pupil planes as a comparison with the classical arrangement will readily show. A factor two to four in the reduction of the chromatic aberration is thus achieved without the presence of a chromatic aberration reducing element, sometimes making further color correction unnecessary. Even a system incorporating several transfers is fully color corrected by the use of a single color correcting element. The distortion, which is usually very high in the objective, is corrected at more convenient and effective places. The result is a single integrated system which replaces the three conventional separate parts, i.e. the objective, the field lens, and a relay lens. This single integrated system may be augmented, as is well known in the art of optical design, with additional optics, like a close-up lens, a field expander, a field flattening lens, or with additional relay groups, without falling outside the scope of the invention.